Homogeneous IgM anti-lactose antibody preparations from the horse will be examined with respect to their specific interactions with antibody. This analysis will include measurement of affinity, variability of the structure of the combining region as revealed by optical probes, the kinetics of interaction, conformational perturbation as seen by low angle X-ray scattering, the quantitative significance of multivalence and the individual contributions of the heavy and light chains. The clonal diversity of the anti-lactose IgM response will be evaluated by fractionation of the antibody into monoclonal fractions. The variety of VH and VL genes expressed in this response as well as their mutational relationship will be examined by sequence analysis of affinity-labeled tryptic peptides. Cellular events initiated by antigen combination with IgM antibody adsorbed to the lymphocyte surface will be identified and quantitated.